dpif: Update documentation for RCU-protected actions.

The userspace datapath returns RCU-protected actions from flow_get() and
flow_dump_next(). This doesn't cause any trouble for current users of
these functions, but it imposes additional constraints on their use.
This patch makes the dpif documentation more explicit about how the
results of these functions can be used.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>